This invention relates to electrostatic printing devices and more particularly to means of projecting toner to an electronically addressable printhead for depositing toner in image configuration on plain paper substrates.
Of the various electrostatic printing techniques, the most familiar and widely used is xerography in which a latent electrostatic image is formed on a charge retentive surface, developed by a suitable toner material to render the image visible, and then transferred to plain paper.
Another form of electrostatic printing is one known as direct electrostatic printing (DEP). In DEP, unlike xerography, toner is deposited directly onto a plain (i.e., not specially treated) substrate in image configuration. This type of printing device is disclosed in U.S. Pat. No. 3,689,935 issued Sep. 5, 1972 to Gerald L. Pressman et al.
Pressman et al. disclose an electrostatic line printer incorporating a multilayered particle modulator or printhead comprising a layer of insulating material, a continuous layer of conducting material on one side of the insulating layer and a segmented layer of conducting material on the other side of the insulating layer. At least one row of apertures is formed through the multilayered particle modulator. Each segment of the segmented layer of the conductive material is formed around a portion of an aperture and is electrically isolated from every other segment of the segmented conductive layer. Selected potentials are applied to each of the segments of the segmented conductive layer while a fixed potential is applied to the continuous conductive layer. An overall applied field projects airborne charged particles through the row of apertures of the particle modulator and the density of the particle stream is modulated according to the pattern of potentials applied to the segments of the segmented conductive layer. The modulated stream of charged particles is intercepted by a print-receiving medium placed in the path of the modulated particle stream and translated relative to the particle modulator to provide line-by-line scan printing. In the Pressman et al. device the supply of the toner to the control member is effected with a uniform field which results in toner accumulations on the printhead, which disturbs the toner flow and produces irregularities in the printed image. High speed recording is difficult and moreover, the openings in the printhead are subject to clogging by the toner.
U.S. Pat. No. 4,491,855 issued on Jan. 1, 1985 in the name of Fujii et al. discloses a method and apparatus utilizing a controller having a plurality of openings or slit-like openings to control the passage of one-component insulative magnetic toner and to record a visible image by the charged particles directly on an image receiving member. Specifically disclosed therein is an improved device in which charged particles are supported on a supporting member and an alternating electric field is applied between the supporting member and the control electrode. Fujii et al. show an apertured printhead structure having wedge-shaped apertures wherein the larger diameter of an aperture is delineated by a signal or control electrode and is disposed opposite an image receiving substrate. Fujii et al. purports to obviate the problems noted above with respect to Pressman et al. and has allegedly made high-speed and stable recording possible.
U.S. Pat. No. 4,568,955 issued on Feb. 4, 1986 to Hosoya et al. discloses a recording apparatus wherein a visible image based on image information as formed on an ordinary sheet by a developer. The recording apparatus comprises a developing roller spaced at a predetermined distance from and facing the ordinary sheet and carrying developer thereon. It further comprises a recording electrode and a signal source connected thereto for propelling the toner on the developing roller to the ordinary sheet by generating an electric field between the ordinary sheet and the developing roller according to the image information. A plurality of mutually insulated electrodes are provided on the developing roller and extend therefrom in one direction. An A. C. and a D.C. source are connected to the electrodes, for generating an alternating electric field between adjacent ones of the electrodes to cause oscillations of the toner found between the adjacent electrodes along electric lines of force there between to thereby liberate the toner from the developer roll. In a modified form of the Hosoya et al. device, a toner reservoir is disposed beneath a recording electrode which has a top provided with an opening facing the recording electrode and an inclined bottom for holding a quantity of toner. In the toner reservoir are disposed a toner carrying plate as the developer carrying member, secured in a position such that it faces the end of the recording electrode at a predetermined distance therefrom and a toner agitator for agitating the toner.
U.S. Pat. No. 4,814,796 granted to Fred W. Schmidlin on Mar. 21, 1989 discloses a direct electrostatic printing (DEP) apparatus including a toner delivery system wherein a donor roller is employed to present charged toner to an apertured printhead, toner being deposited on the donor roller via a magnetic brush structure. The donor roller is positioned adjacent the printhead structure to form a nip area therebetween. The toner on the donor roller is excited into a cloud-like state in the nip area via an A. C. voltage applied between the donor roller and the shield electrode of the apertured printhead. A two-component magnetic brush is used to deposit toner on the donor roller because the toner charge distribution of the charged toner is most nearly unipolar in magnetic brushes. In operation of the DEP apparatus, the toner predominantly charged to the one polarity, referred to as the right sign toner, is passed through an aperture and deposited on the receiver substrate, such as plain paper, to print black. The control electrode and the paper shoe are set to voltages opposite in polarity to the charge on the right sign toner. The voltage of the paper shoe is made to be much greater than the voltage on the control electrode so the right sign toner is attracted to the paper shoe and not to the control electrode. To stop the passage of toner through an aperture the control electrode is switched to a large voltage the same polarity as the right sign toner. This repels the right sign toner and forces it back toward the donor. Under these conditions no toner deposits on the paper and it remains white. The control electrode is then said to be in the OFF state. In this OFF state any toner in the toner cloud in front of the aperture which is opposite in polarity to the right sign toner, referred to as the wrong sign toner (WST), will be drawn through the aperture and collected on the control electrode. The WST does not deposit on the paper because the paper shoe is the same polarity as the WST and therefore repels the WST away from the paper. Thus, collection of WST on the control electrode does not directly impact image quality and might not even be recognized as a problem. The presence of WST on a control electrode manifests itself as a problem when an aperture is maintained in the OFF condition for extended periods of time, as needed to print large white areas. Then, relatively large amounts of WST accumulate on the control electrodes and the electrostatic charge associated with these accumulations produces an electric field that counters the working field produced by the control voltage. Eventually, this counter field negates enough of the control field to enable right sign toner to leak through the aperture. This toner does land on the paper, where it produces a noticeable, unwanted, gray background.
The foregoing discussion explains the fundamental reason why DEP requires the use of a magnetic brush which contains a very low concentration of WST. With a sufficiently low amount of WST in the toner supply it is possible to maintain a control electrode in the OFF state for one page length without producing a noticeable level of gray background. The printhead can then be restored to a clean state between pages using a cleaning process such as that described in U.S. Pat. No. 4,755,837, invented by Fred W. Schmidlin et al..
By way of example, a DEP apparatus designed to work with negative toner may utilize a paper shoe set to +400 volts and control electrodes biased to +50 Volts in the ON state and -350 volts in the OFF state. In this case, the positive wrong sign toner will be repelled from the paper shoe and attracted to the negative control electrode in the OFF state. With these operating voltages it is known that an 11 inch length of white with no noticeable background can be printed if the quantity of wrong sign toner that flows to the control electrodes in the OFF state is less than 0.2% of the right sign toner that flows to the paper in the ON state.
It may be appreciated from the above example that the tolerance of DEP for wrong sign toner is extremely low. It may also be appreciated that the required level of wrong sign toner is achieved only with the highest quality of two-component xerographic developers such as that found in Xerox 9200 laser printers. It is also known that the proportion of wrong sign toner present in a magnetic brush developer increases with use. Thus the length of time a magnetic brush can be used to deposit toner on a DEP donor roll is severely limited. Therefore it would be a great advantage to have a direct printing process which is much less sensitive to the presence of wrong sign toner, a process which could use a wider variety of xerographic developers with long life, and would eliminate the need to clean the printhead after every page.